When working in a condition with low light or even no light, a user cannot see the alphanumeric symbols on the keys of a keyboard clearly, for example, when a notebook computer is used on airplanes, or when a keyboard attached to equipment is used in a lithography laboratory.
FIG. 1 is an upper view diagram of an illuminating keyboard disclosed in U.S. Pat. No. 6,179,432. The keyboard 10 has a plurality of key caps 11 mounted on a housing 15 and is allowed to have finite downward displacement along its vertical direction. There is a symbol 12 printed on the surface of each of the key caps 11 to illustrate the function of the corresponding one, for example, numeric input keys 0–9, alphabetic input keys A–Z and function keys F1–F12. An electroluminescent device 13 is placed under the key caps 11, and is driven to illuminate by an AC source. Therefore, the 3.3V or 5V DC power supplied by a cable 14 needs to be converted into the specific AC power so as to meet the requirement of the electroluminescent device 13.
FIG. 2 is an exploded diagram of the illuminating keyboard in FIG. 1. The electroluminescent device 13 comprises a plurality of openings 131, each of which is aligned with the corresponding key cap 11 above. Moreover, a driving circuit 133 converts the 3.3V or 5V DC power supplied by a cable 14 into the specific AC power so as to meet the specification of the electroluminescent device 13. An elastomer layer 21 made from rubber or silicone is placed beneath the electroluminescent device 13, and has a plurality of key actuators 211 whose positions correspond to the positions of the key caps 12. Because the key actuator 211 is very flexible, it can rapidly return to its original shape after a keystroke is released. When the key actuator 211 is deformed after the pressing of the key cap 12, a membrane switch 22 beneath the elastomer layer 21 is switched on, and meanwhile an input signal corresponding to the key cap 12 is transmitted to a host through the cable 14.
Because the electroluminescent device 13 is placed beneath the key cap 12, the structure of the key cap 12 needs to be redesigned to hold the thickness of the electroluminescent device 13. On the other hand, the deformed movement is still necessary for the key actuator 211, and hence the total thickness of the key cap 12 is increased and the design of the keyboard structure becomes difficult.